1. Field of the Invention
The present invention relates to an improved design of a piezoelectric positioning device for scanning probe microscopes to control the relative position of the surface of a sample under observation to a tip member. Specifically, the present invention relates to an improved electrode arrangement for a piezoelectric device to eliminate tilting of the device during scanning.
2. Description of the Prior Art
In general, a scanning probe microscope operates by positioning a conducting tip or needle either just touching or very close to the surface of a conducting sample. In particular, if the scanning probe microscope is a scanning tunneling microscope, the relative position between the tip and the surface is approximately ten angstroms (10 .ANG.) above the surface of the sample and as described in the prior art, a tunneling current flowing between the sample and the tip is used to detect the profile of the surface. If the scanning probe microscope is an atomic force microscope then the tip just touches the surface of the sample and detection of the position of the tip is produced as provided in the prior art.
In general, scanning probe microscopes include positioning apparatus so as to control the relative scan between the tip and the surface of the sample in an X-Y raster scan while keeping the tip just in contact with the surface of the sample or a substantially constant height above the surface. In both types of scanning probe microscopes it is possible to, either keep the tip stationary and move the sample with the positioning device in order to perform the scan, or to keep the sample stationary and move the tip with the positioning device in order to perform the scan. In general, although this is not necessary, a scanning tunneling microscope normally has the sample stationary with the tip movable whereas an atomic force microscope normally has the tip stationary and the sample movable relative to the tip.
Our prior art U.S. Pat. No. 4,871,938 issued on Oct. 3, 1989 was directed to a specific problem relating to the nonperpendicular nature of the X-Y scan for positioning devices prior to the '938 patent and also for minimizing horizontal-vertical cross-coupling and in particular for minimizing the vertical-horizontal cross-coupling at the side of the tube where the tip was mounted.
In our prior art design, and in particular the embodiment which used a cylindrical positioning device, a "pentrode" design was used which kept the side of the tube on which the tip was mounted from moving up and down as the tube was scanned side to side. Although this design was excellent for a tip that is mounted at one point on the scanner, the design does not work well when the sample is mounted on the end of the positioning device. In our previous design the tube tilts as the sample is scanned in the X and Y directions. As indicated above, mounting the sample on the end of the positioning device is the preferred form for an atomic force microscope, but is to be appreciated that a scanning tunneling microscope may also be operated in the same way.